Edge-emitting semiconductor laser

ABSTRACT

An edge emitting semiconductor laser comprising an active, radiation-generating zone ( 1 ), and an common waveguide ( 8 ), which is suitable for guiding the radiation generated in the active zone ( 1 ) within the semiconductor laser. The common waveguide ( 8 ) comprises a first n-doped layer ( 4 ) and a second n-doped layer ( 5 ), which is arranged between the first n-doped layer ( 4 ) and the active zone ( 1 ), wherein the refractive index n 2  of the second n-doped layer ( 5 ) is greater than the refractive index n 1  of the first n-doped layer ( 4 ) by a value dn.

RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/EP2010/062812,filed on Sep. 1, 2010.

This patent application claims priority of German Patent Application 102009 041 934.9 filed Sep. 17, 2009, the disclosure content of which ishereby incorporated by reference.

The present application relates to an edge emitting semiconductor laser.

BACKGROUND OF THE INVENTION

Edge emitting semiconductor lasers generally have a waveguide in orderto guide the radiation generated in an active zone of the semiconductorlaser within the semiconductor laser.

Conventional waveguides generally comprise p-doped semiconductor layers,which can lead to a high series resistance of the device on account ofthe low hole mobilities. In order to reduce the series resistance ofsuch a component, it has been attempted, for example, to increase thep-type doping of the p-doped region of the waveguide. In that case,however, there is the risk of the losses of the waveguide increasingwhich can result in an increase in the threshold current of the laser.

SUMMARY OF THE INVENTION

An object of the present invention is to specify an edge emittingsemiconductor laser comprising an improved waveguide which enables a lowseries resistance of the semiconductor laser and enables very highefficiency at high currents.

An edge emitting semiconductor laser comprises, in particular:

-   -   an active, radiation-generating zone, and    -   a common waveguide, which is suitable for guiding the radiation        generated in the active zone within the semiconductor laser,        wherein    -   the common waveguide comprises a first n-doped layer and a        second n-doped layer, wherein the second n-doped layer is        arranged between the first n-doped layer and the active zone,        and    -   the refractive index n₂ of the second n-doped layer is greater        than the refractive index n₁ of the first n-doped layer by a        value dn.

The term confinement factor (CF) hereinafter denotes the overlap of therespective laser mode of the semiconductor laser with theradiation-generating active zone of the semiconductor laser.Furthermore, the first-order laser mode is also designated by the term“fundamental laser mode”.

The common waveguide has the task of increasing the confinement factorof the fundamental laser mode relative to the confinement factors ofhigher-order laser modes. In the present case, the common waveguidecomprises a primary waveguide and also at least one further,higher-order waveguide, generally a secondary waveguide.

The primary waveguide guides the fundamental mode. In this case, theterm “guiding” means that at least 50% of the total intensity of themode is concentrated within the respective waveguide.

The primary waveguide preferably comprises the active zone and thesecond n-doped layer. The primary waveguide can furthermore comprise aso-called secondary well. A secondary well is formed, for example, by anundoped region that embeds the active zone. The undoped region generallyhas a higher refractive index than the layers of the common waveguide.

The secondary waveguide comprises the primary waveguide and, adjoiningthe second n-doped layer, the first n-doped layer, wherein therefractive index n₂ of the second n-doped layer is greater than therefractive index n₁ of the first n-doped layer by a value dn. Thesecondary waveguide is provided for guiding at least the second-ordermode, if no higher-order waveguide is provided, all higher-order modes.

Even higher waveguides for guiding higher-order modes are alsoconceivable. The material composition and thickness of the respectivelayers which form the respective waveguide have to be chosen in such away that the effective refractive index of the fundamental mode is highenough in order that the fundamental mode is guided in the primarywaveguide but not in the secondary waveguide and—if present—in higherwaveguides.

It should be pointed out at this juncture that the common waveguide cancomprise further layers alongside the layers mentioned above. Thesefurther layers can be part of the primary waveguide, of the secondarywaveguide or of a higher-order waveguide.

One aim of the present invention is to have a favourable influence onthe confinement factors of the different modes by introducing primaryand higher waveguides which guide the different laser modes differently.In particular, the present invention strives to increase the confinementfactor of the fundamental laser mode relative to the confinement factorof higher-order laser modes.

In accordance with one particularly preferred embodiment of thesemiconductor laser, the common waveguide is free of a p-doped layer.

Where mention is made of n-doped or p-doped layers in the present case,this is taken to mean that these layers have corresponding additionaldopants (n- or p-type dopants). In the present case, undoped layers areunderstood to be layers into which no additional dopants are introducedin a targeted manner. However, such undoped layers may very well have anintrinsic doping, which arises unavoidably during the deposition of thelayers, for example as a result of epitaxial growth in a reactor.

In this sense, the fact that the common waveguide is “free of a p-dopedlayer” means that the common waveguide has no layer that hasadditionally been doped with a p-type dopant in a targeted manner.

In accordance with a further embodiment, the common waveguide of thesemiconductor laser has only one single very thin p-doped layer.Particularly preferably, this very thin p-doped layer has a thicknessthat is less than or equal to 10% of the thickness of the commonwaveguide, for example less than or equal to 200 nm. It should bepointed out at this juncture that the p-doped layer need not necessarilybe embodied homogeneously with regard to the material. Rather, thematerial composition can vary within the p-doped layer.

If the common waveguide has no or only very thin p-doped layers, thenthis has the advantage that the series resistance of the semiconductorlaser is reduced by comparison with a conventional semiconductor laserhaving thicker p-doped waveguide layers with the same common thicknessof the common waveguide.

Particularly preferably, the fundamental mode of the laser radiation issubstantially limited to the second n-doped layer. This means in thepresent case that more than half of the intensity is concentrated in thesecond n-doped layer and the active zone.

In accordance with a further preferred embodiment of the semiconductorlaser, the common waveguide furthermore has a first undoped layer, whichis arranged between the active zone and the second n-doped layer,wherein the refractive index n₃ of the first undoped layer is greaterthan the refractive index n₂ of the second n-doped layer.

In this embodiment, a second undoped layer is furthermore preferablyarranged within the common waveguide, said layer being arranged on theside of the active zone which lies opposite the first undoped layer. Inthis case, the active zone is embedded within the two undoped layers.The two undoped layers form an undoped region that can form a secondarywell.

Furthermore, in this embodiment wherein the common waveguide has,alongside the first n-doped layer and the second n-doped layer, twoundoped layers into which the active zone is embedded, it is possiblefor the first undoped layer and the second undoped layer to havedifferent thicknesses. In this case, the active zone is thus arrangedasymmetrically within an undoped region formed by the first and secondundoped layers. Particularly preferably, in this case, the first undopedlayer, which is arranged between the active zone and the second n-dopedlayer, is made thinner than the second undoped layer, which is arrangedon the side of the active zone which is remote from the first undopedlayer. As a result, the confinement factor of the fundamental mode canadvantageously be increased further.

In accordance with a further embodiment of the semiconductor laser, thecommon waveguide additionally has an n-doped cladding layer and ap-doped cladding layer. A layer is designated as “cladding layer” in thepresent case if its refractive index is lower than the effectiverefractive index of the mode currently under consideration.

Generally, the semiconductor layers of the semiconductor laser, such asthe active zone, the first undoped layer, the second undoped layer, thefirst n-doped layer, the second n-doped layer, but also the claddinglayers, are grown epitaxially on a growth substrate. The growthsubstrate can comprise one of the following materials or consist of oneof the following materials: gallium arsenide, indium phosphide, indiumarsenide, indium antimonide, gallium antimonide, gallium nitride. If thegrowth substrate has a higher refractive index than that layer of thecommon waveguide which is closest to the growth substrate, then ann-doped cladding layer or a p-doped cladding layer is preferablyarranged between the common waveguide and the growth substrate. In thiscase, the doping of the cladding layer is generally oriented towards thedoping of the adjacent layers of the common waveguide, that is to saythat an n-doped cladding layer is preferably arranged between the growthsubstrate and the common waveguide if that layer of the common waveguidewhich is arranged adjacent to the growth substrate is n-doped. If thatlayer of the common waveguide which is arranged adjacent to the growthsubstrate is p-doped, however, then a p-doped cladding layer isgenerally positioned between the growth substrate and the commonwaveguide.

By means of the p-doped cladding layer or the n-doped cladding layerbetween the growth substrate and the common waveguide, the modes of thelaser radiation are advantageously effectively decoupled from the growthsubstrate. The efficiency of the semiconductor laser is therebyincreased since the modes of the laser radiation would otherwiseexperience high optical absorption losses and/or the vertical far fieldof the semiconductor laser can be disturbed.

In this case, generally the n-doped cladding layer is arranged adjacentto the first n-doped layer of the common waveguide, while the p-dopedcladding layer is arranged on the side of the active zone which liesopposite the n-doped layers.

In accordance with a further preferred embodiment, the confinementfactor of the fundamental mode of the semiconductor laser per quantumwell is between 1% and 0.4%, and particularly preferably between 0.6%and 0.4%, in each case inclusive of the limits.

In accordance with a further embodiment, the refractive index differencedn between the first n-doped layer and the second n-doped layer isbetween 0.03 and 0.1, inclusive of the limits. If the refractive indexdifference dn is relatively small, then the fundamental mode alsopropagates within the first n-doped layer, as a result of which thewidth of the far field of the semiconductor laser is advantageouslysignificantly reduced.

The thickness of the second n-doped layer is furthermore preferablybetween 200 nm and 800 nm, once again inclusive of the limits.

Particularly preferably, the thickness of the second n-doped layer isbetween 8% and 20% relative to the thickness of the common waveguide.

In accordance with a further embodiment, the concentration of the n-typedopant within the first n-doped layer exhibits a profile which increasesin a stepped manner or continuously towards the outer side of the commonwaveguide. In this way, it is generally possible to bring about anincrease in optical losses on account of the absorption of free chargecarriers for higher-order laser modes in order to suppress the formationof higher-order modes alongside the fundamental laser mode within thesemiconductor laser.

In accordance with a further embodiment, the edge emitting semiconductorlaser is based on an arsenide compound semiconductor material.

In this context, “based on arsenide compound semiconductor material”means that, in particular, the radiation-generating active zone of theedge emitting semiconductor laser preferably comprisesAl_(n)Ga_(m)In_(1-n-m)As, where 0≦n≦1, 0≦m≦1 and n+m≦1. In this case,this material need not necessarily have a mathematically exactcomposition according to the above formula. Rather, it can comprise oneor more dopants and additional constituents which, however, do notessentially change in particular the refractive index of the material.For the sake of simplicity, however, the above formula only includes theessential constituents of the crystal lattice (Al, Ga, In, As), even ifthese can be replaced in part by small amounts of further substances.

A semiconductor laser based on an arsenide compound semiconductormaterial is suitable, in particular, for emitting laser radiation fromthe infrared spectral range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional illustration of an edge emittingsemiconductor laser in accordance with a first exemplary embodiment,

FIG. 2 shows a schematic illustration of the refractive index n of theedge emitting semiconductor laser in accordance with the exemplaryembodiment from FIG. 1 along the direction R as a function of thedistance b,

FIG. 3 shows a schematic illustration of the refractive index profile ofthe semiconductor laser in accordance with the exemplary embodiment fromFIG. 1 with the intensity profiles I of the optical modes up to thefourth order within the common waveguide,

FIG. 4 shows a schematic illustration of the confinement factor CF fordifferent modes of the semiconductor laser in accordance with theexemplary embodiment from FIG. 1 as a function of the thickness d of thesecond n-doped layer,

FIG. 5 shows a schematic illustration of the refractive index profile ofthe semiconductor laser in accordance with the exemplary embodiment fromFIG. 1 and also three different profiles of the n-type dopantconcentration c_(n) along the direction R in accordance with threedifferent exemplary embodiments,

FIG. 6 shows a schematic illustration of the refractive index profile nof an edge emitting semiconductor laser in accordance with a secondexemplary embodiment with the intensity profiles of the optical modes upto the fourth order within the common waveguide,

FIG. 7 shows a schematic illustration of the confinement factor CF fordifferent modes of the semiconductor laser in accordance with the secondexemplary embodiment as a function of the thickness d of the secondn-doped layer,

FIG. 8 shows a schematic illustration of the refractive index profile nof an edge emitting semiconductor laser in accordance with a thirdexemplary embodiment with the intensity profiles of the optical modes upto the fourth order within the common waveguide, and

FIG. 9 shows a schematic illustration of the confinement factor CF fordifferent modes of the semiconductor laser in accordance with the thirdexemplary embodiment as a function of the thickness d of the secondn-doped layer.

DETAILED DESCRIPTION OF THE DRAWINGS

Identical or identically acting elements are provided with the samereference symbols in the figures. The figures should not be regarded astrue to scale; rather, individual elements, for example layerthicknesses, may be illustrated with an exaggerated size forclarification purposes.

The edge emitting semiconductor laser in accordance with the exemplaryembodiment in FIG. 1 has an active zone 1, which is suitable forgenerating electromagnetic radiation.

For the purpose of generating radiation, the active zone 1 preferablyhas a pn junction, a double heterostructure, a single quantum well or amultiple quantum well structure (MQW). Particularly preferably, theactive zone 1 has a single quantum well for generating radiation. Inthis case, the designation quantum well structure does not include anyindication about the dimensionality of the quantization. It thereforeencompasses, inter alia, quantum wells, quantum wires and quantum dotsand any combination of these structures.

In the case of the semiconductor laser in accordance with the exemplaryembodiment in FIG. 1, the active zone 1 is arranged between a firstundoped layer 2 and a second undoped layer 3. In this case, the firstundoped layer 2 and the second undoped layer 3 are arranged in a mannerdirectly adjoining the active zone 1, that is to say that the firstundoped layer 2 and the second undoped layer 3 in each case form acommon interface with the active zone 1. The active zone 1 is thusembedded into an undoped region formed by the first undoped layer 2 andthe second undoped layer 3. The first undoped layer 2 and the secondundoped layer 3 form an undoped region that acts as a secondary well.

Furthermore, the semiconductor laser in accordance with FIG. 1 comprisesa first n-doped layer 4 and a second n-doped layer 5, wherein the secondn-doped layer 5 is arranged between the active zone 1 and the firstn-doped layer 4. In the present case, the second n-doped layer 5 isarranged in direct contact with the first undoped layer 2 and the firstn-doped layer 4 is arranged in direct contact with the second n-dopedlayer 5, that is to say that the second n-doped layer 5 forms a commoninterface with the first undoped layer 2 and the first n-doped layer 4forms a common interface with the second n-doped layer 5.

In the present case, the active zone 1, the two undoped layers 2, 3 andthe two n-doped layers 4, 5 form the common waveguide 8 of thesemiconductor laser, which is suitable for guiding the radiationgenerated in the active zone 1 within the semiconductor laser. In thepresent case, the active zone 1 is arranged within the common waveguide8, to be precise symmetrically between the two undoped layers 2, 3 thatform the undoped region of the common waveguide 8. In other words, thetwo undoped layers 2, 3 have the same thickness.

Furthermore, an n-doped cladding layer 6 is arranged adjacent to thefirst n-doped layer 4, while a p-doped cladding layer 7 is positionedadjacent to the second undoped layer 3 on that side of the active zone 1which lies opposite the n-doped cladding layer 6. In the present case,the p-doped cladding layer 7 is arranged in direct contact with thesecond undoped layer 3, that is to say that the p-doped cladding layer 7forms a common interface with the second undoped layer 3.

The layers of the common waveguide 8, that is to say in the present casethe active zone 1, the first undoped layer 2, the second undoped layer3, the first n-doped layer 4, the second n-doped layer 5, are grownepitaxially on a growth substrate 13. The layers of the common waveguide8 are separated from the growth substrate 13 by an n-doped claddinglayer 6.

The common waveguide 8 of the semiconductor laser in accordance withFIG. 1 is free of a p-doped layer. Its series resistance is thereforeadvantageously reduced by comparison with a component having p-dopedlayers within the common waveguide 8. The series resistance of thesemiconductor laser relative to the area is preferably less than orequal to 2*10⁻⁴ Ωm², particularly preferably less than or equal to 10⁻⁵Ωcm².

The semiconductor laser has facets at its two side areas 9, 10 situatedperpendicular to the main extension plane of the active zone 1, saidfacets being suitable for reflecting the radiation generated in theactive zone 1. Therefore, during the operation of the semiconductorlaser, along the main extension plane of the active zone 1, a standingwave of electromagnetic radiation forms within the semiconductor laser,which constitutes the laser radiation. This laser radiation is generallycoupled out through one of the facets.

FIG. 2 illustrates the profile of the refractive index n within thesemiconductor laser along the direction R in accordance with FIG. 1 as afunction of the thickness b of the layers of the semiconductor laser. Inthis case, the refractive index profile begins with the refractive indexof the growth substrate 13, which is approximately 3.6. Arranged in amanner adjoining the growth substrate 13 is the n-doped cladding layer6, which has a thickness of between approximately 0.7 and approximately1 μm and a refractive index of approximately 3.2. In the present case,the n-doped cladding layer 6 has the task of decoupling the modes of thelaser radiation from the growth substrate 13.

The first n-doped layer 4, which is situated in direct contact with then-doped cladding layer 6, has a thickness of approximately 1.8 μm and arefractive index n₁ of approximately 3.37. Furthermore, arranged in amanner directly adjoining the first n-doped layer 4 is the secondn-doped layer 5, which has a thickness of approximately 0.35 μm and arefractive index n₂ of approximately 3.41.

The refractive index n₂ of the second n-doped layer 5 is thus greaterthan the refractive index n₁ of the first n-doped layer 4 by a value dnof approximately 0.04.

Particularly preferably, the refractive index difference dn between thefirst n-doped layer 4 and the second n-doped layer 5 is between 0.04 and0.05, inclusive of the limits.

The first undoped layer 2 and the second undoped layer 3 both have athickness of between 10 nm and 100 nm and a refractive index ofapproximately 3.45. The refractive index n₃ of the first undoped layer 2is thus greater than the refractive index n₂ of the second n-doped layer5. The active zone 1, which is embedded into the two undoped layers 2,3, has a refractive index of approximately 3.6. The thickness of theactive zone 1 is generally between 3 nm and 12 nm, for exampleapproximately 7 nm.

In the present case, the p-doped cladding layer 7, which is arranged ina manner directly adjoining the second undoped layer 3, has a thicknessof approximately 0.7 μm and a refractive index of approximately 3.2.

In the present case, the semiconductor laser is based on an arsenidecompound semiconductor material and is suitable for emitting laserradiation from the infrared spectral range with a wavelength ofapproximately 965 nm.

The present concept for an common waveguide of an edge emittingsemiconductor laser is, however, not restricted to this material groupor to this emission wavelength. Rather, the present concept can also beapplied to other materials and emission wavelengths, for example 808 nm.

The n-doped cladding layer 6 and the p-doped cladding layer 7, the firstn-doped layer 4 and the second n-doped layer 5 and also the firstundoped layer 2 and the second undoped layer 3 comprise AlGaAs havingdifferent aluminium contents. In this case, the different aluminiumcontents of the layers lead to the above-described differences in therefractive indices of the layers.

The p-doped cladding layer 7 and the n-doped cladding layer 6 have analuminium content of approximately 50%, while the aluminium content ofthe two undoped layers 2, 3 is approximately 11%. The first n-dopedlayer 4 has an aluminium content of approximately 25%, while the secondn-doped layer 5 has an aluminium content of approximately 19%.

In the present case, the active zone 1 has InGaAs quantum structures forgenerating radiation. In contrast to the rest of the layers, the activezone 1 contains no aluminium in the present case. However, it should bepointed out that the active zone 1 generally comprises aluminium inparticular for emission wavelengths of less than 850 nm.

FIG. 3 shows the refractive index profile of the semiconductor laser inaccordance with FIG. 1 such as has already been described with referenceto FIG. 2. Furthermore, the intensities I₁, I₂, I₃, I₄ of the modes ofthe laser radiation which form within the common waveguide 8 up to thefourth order and also the effective refractive indices n_(eff1),n_(eff2), n_(eff3), n_(eff4), thereof are plotted in FIG. 3.

In this case, the fundamental mode of the laser radiation, that is tosay the first-order mode, is localized distinctly within the secondn-doped layer 5, that is to say that at least half of the intensity ofthe fundamental mode is localized within the second n-doped layer 5. Inthis case, the intensity profile I₁ of the fundamental mode has a singlemaximum having a normalized value of approximately 1.6, which issituated within the second n-doped layer 5. In this case, the intensityI₁ falls from its maximum steeply to the undoped layers 2, 3 and theactive zone 1 and exhibits a small falling tail into the p-dopedcladding layer 7. The intensity of the fundamental mode I₁ falls moreshallowly on that side of the maximum which is remote from the activezone 1. It extends through the first n-doped layer 4. The intensity ofthe fundamental mode within the n-doped cladding layer 6 is negligiblysmall.

In the present case, the active zone 1, the first undoped layer 2, thesecond undoped layer 3 and the second n-doped layer 5 form a primarywaveguide 81 suitable for guiding the fundamental laser mode.

The intensities I₂, I₃ and I₄ of the second-, third- and fourth-ordermodes likewise have a local maximum within the second n-doped layer 5near the undoped layers 2, 3, but said local maximum is in each casedistinctly smaller than the value of the maximum of the fundamentalmode. The second-order mode here has a maximum of approximately 0.2,while the maximum of the third-order mode is approximately 0.4 and themaximum of the fourth-order mode is approximately 0.5.

In the present case, the active zone 1, the first undoped layer 2, thesecond undoped layer 3, the first n-doped layer 4 and the second n-dopedlayer 5 form a secondary waveguide 82 suitable for guiding higher-orderlaser modes.

The common waveguide 8 comprises the layers of the primary waveguide 81and the layers of the secondary waveguide 82. In the present case, thecommon waveguide 8 is formed by the active zone 1, the first undopedlayer 2, the second undoped layer 3, the first n-doped layer 4 and thesecond n-doped layer 5.

Since the higher-order modes, in particular the second- and third-ordermodes, have a local maximum within the second n-doped layer 5 which isdistinctly smaller than the maximum of the fundamental mode, theprobability of the occurrence of higher-order modes, in particular ofthe second- or third-order mode, is advantageously low. Higher-ordermodes, generally starting from the fourth-order mode, often exhibittails into the cladding layers, in which they experience high opticallosses on account of the higher doping. Consequently, the occurrence ofhigher-order modes is generally rather low.

The intensity I₂ of the second-order mode furthermore has a singlefurther local maximum within the first n-doped layer 4 at a value ofapproximately 1.0 and a position b of approximately 2.75 μm. In thiscase, the width of this further maximum is approximately 1.6 μm.

The intensity I₃ of the third-order mode has two further local maximaeach having a value of approximately 0.8, which are likewise positionedwithin the first n-doped layer 4. In this case, the width of these twomaxima is approximately 1 μm. The maxima are arranged symmetrically withrespect to the local minimum of the curve at a position b=2.9 μm.

The intensity I₄ of the fourth-order mode has three further local maximaeach having a value of approximately 0.7. These further local maxima ofthe intensity I₄ of the fourth-order mode are also positioned within thefirst n-doped layer 4. In this case, the width of these three maxima isapproximately 0.7 μm and the maxima are arranged symmetrically withrespect to the maximum of the average maximum at a position b=2.9 μm.

As a consequence of the intensity profile of the different modes withinthe semiconductor laser, the values of the effective refractive indicesn_(eff1), n_(eff2), n_(eff3), n_(eff4) decrease as the mode orderincreases. The effective refractive index n_(eff1) of the fundamentalmode is approximately 3.38, the effective refractive index n_(eff2) ofthe second-order mode is approximately 3.36, the effective refractiveindex n_(eff3) of the third-order mode is approximately 3.34 and theeffective refractive index n_(eff4) of the fourth-order mode isapproximately 3.33.

FIG. 4 shows the confinement factor CF₁, CF₂, CF₃, CF₄ of thefundamental laser mode and of the second-, third- and fourth-order modesas a function of the thickness d of the second n-doped layer 5 for theexemplary embodiment in accordance with FIGS. 1, 2 and 3. In this case,the total thickness of the common waveguide 8 is kept constant. It isapproximately 2.3 μm. The confinement factor CF₁ of the fundamentallaser mode has a maximum value of approximately 0.55% with a thickness dof the second n-doped layer 5 of approximately 0.3 μm, while the maximumvalues of the confinement factors CF₂, CF₃, CF₄ of the second-, third-and fourth-order laser modes are shifted distinctly to higher values ofthe thickness d of the second n-doped layer 5. Furthermore, theconfinement factor CF₁ of the fundamental laser mode has, at the originof the graph, that is to say for the case where the common waveguide 8is free of a second n-doped layer 5, a value that is lowered bycomparison with the values of the confinement factor of the second-,third- and fourth-order modes. This means that, in a conventional commonwaveguide having only a single n-doped layer rather than two n-dopedlayers 4, 5, as is provided in the present case, the confinement factorCF₁ of the fundamental laser mode is lower than the confinement factorsCF₂, CF₃, CF₄ of the higher-order modes, such that the risk of theoccurrence of undesired higher-order laser modes is increased.

FIG. 5 shows the profile of the refractive index n of the edge emittingsemiconductor laser in accordance with the exemplary embodiment fromFIG. 1 such as has already been described in detail with reference toFIG. 2. Furthermore, FIG. 5 schematically illustrates a first n-typedoping profile c_(n) within the semiconductor laser. As an alternativethereto, two further modified n-type doping profiles c_(n1) and C_(n2)are illustrated schematically, in the case of which the doping increasestowards the side of the first n-doped layer 4 which is remote from theactive zone 1. The side of the first n-doped layer 4 which is remotefrom the active zone 1 forms an outer side of the common waveguide 8.

In this case, by way of example, tellurium or silicon can be used asn-type dopants.

The first n-type doping profile C_(n) has a concentration of the n-typedopant of approximately 1 10¹⁷ cm⁻³ within the n-doped cladding layer 6and falls to a value of approximately 1 10¹⁶ cm⁻³ at the transition tothe first n-doped layer 4. The first n-doped layer 4 and the secondn-doped layer 5 have the same n-type dopant content of 1 10¹⁶ cm⁻³, thatis to say that the first n-type doping profile C_(n) proceeds in aconstant fashion at a value of approximately 1 10¹⁶ cm⁻³ within thefirst n-doped layer 4 and the second n-doped layer 5. At the transitionto the undoped region formed by the two undoped layers 2, 3, the firstn-type doping profile C_(n) falls sharply towards zero.

In contrast to the first n-type doping profile c_(n), the first modifiedn-type doping profile c_(n1) rises continuously within a thickness ofthe first n-doped layer 4 of approximately 0.5 μm to the side of thefirst n-doped layer 4 which is remote from the active zone 1. In thiscase, the first modified n-type doping profile has a profile inaccordance with an inverted parabola.

The second modified n-type doping profile C_(n2) rises in the same wayas the first modified n-type doping profile within a thickness ofapproximately 0.5 μm of the first n-doped layer 4 to the side of thefirst n-doped layer 4 which is remote from the active zone 1. Incontrast to the first modified n-type doping profile c_(n1), however,the second n-type doping profile C_(n2) proceeds in accordance with arectangular function with a value of approximately 2 10¹⁶ cm⁻³.

Since the fundamental laser mode is largely limited to the secondn-doped layer 5 and the first undoped layer 2, a modified n-type dopingprofile, as described by way of example with reference to FIG. 5, canadvantageously be used to increase the losses of the higher-order modeson account of absorption of free charge carriers.

The semiconductor laser in accordance with the second exemplaryembodiment in FIGS. 6 and 7 has, in contrast to the semiconductor laserin accordance with the first exemplary embodiment in FIGS. 1 to 3, athird n-doped layer 11 arranged in a manner adjoining the first n-dopedlayer 4. In order to avoid repetition, in particular the differences ofthe semiconductor laser in accordance with the second exemplaryembodiment in comparison with the semiconductor laser in accordance withthe first exemplary embodiment are described below.

The thickness of the common waveguide 8 in accordance with the secondexemplary embodiment is kept constant by comparison with the firstexemplary embodiment. The third n-doped layer 11 has a thickness ofapproximately 0.5 μm, while the first n-doped layer 4 has a thickness of1.5 μm.

The third n-doped layer 11 is arranged in direct contact with the firstn-doped layer 4. The refractive index n₃ of the third n-doped layer 11is lower than the refractive index n₁ of the first n-doped layer 4 by asecond refractive index difference dn₂.

Particularly preferably, the second refractive index difference dn₂between the first n-doped layer 4 and the third n-doped layer 5 islikewise between 0.04 and 0.05, inclusive of the limits.

With the aid of the third n-doped layer 11 and the second refractiveindex difference dn₂ between the first n-doped layer 4 and the thirdn-doped layer 11, a further higher-order waveguide 83, namely athird-order waveguide, is provided in the present case. The third-orderwaveguide 83 contributes to guiding the higher-order modes.

In the present case, the third-order waveguide 83 comprises the activezone 1, the first undoped layer 2, the second undoped layer 3, the firstn-doped layer 4, the second n-doped layer 5 and the third n-doped layer11.

The second refractive index difference dn₂ between the first n-dopedlayer 4 and the third n-doped layer 11 leaves the intensity profile I₁of the fundamental mode substantially unchanged, while the intensityprofiles I₂, I₃, I₄ of the higher-order modes experience a slightchange. Thus, by way of example, the intensity profile I₂ of thesecond-order mode has a slightly higher maximum by comparison with thefirst exemplary embodiment (cf. FIG. 3).

FIG. 7 shows, in a manner equivalent to FIG. 4, the confinement factorsCF₁, CF₂, CF₃, CF₄ of the fundamental laser mode and of the second-,third- and fourth-order modes as a function of the thickness d of thesecond n-doped layer 5 for the exemplary embodiment in accordance withFIG. 6. In this case, the thickness of the third n-doped layer 11 ismodified in such a way that the total thickness of the common waveguide8 is kept constant.

As shown in FIG. 7, a third n-doped layer 11 with a further refractiveindex difference dn₂ can be introduced into the semiconductor laserstructure in accordance with FIGS. 1, 2 and 3 without the performance ofthe semiconductor laser changing significantly.

Furthermore, FIG. 7 shows that the confinement factors CF₂, CF₃, CF₄ ofthe higher modes with a thickness of the second n-doped layer 5 ofapproximately 300 nm are still suppressed relative to the confinementfactor CF₁ of the fundamental mode.

The semiconductor laser in accordance with the third exemplaryembodiment in FIGS. 8 and 9 has, in contrast to the semiconductor laserin accordance with the first exemplary embodiment in FIGS. 1 to 3, avery thin p-doped layer 12 within the common waveguide 8.

In order to avoid repetition, in particular the differences of thesemiconductor laser in accordance with the third exemplary embodiment incomparison with the semiconductor laser in accordance with the firstexemplary embodiment are described below.

The p-doped layer 12 is arranged between the p-doped cladding layer 7and the second undoped layer 3. It has approximately the same refractiveindex as the second n-doped layer 5. The p-doped layer 12 isparticularly preferably not thicker than 10% of the thickness of thecommon waveguide 8. In the present case, the p-doped layer 12 has athickness of approximately 50 nm.

If the intensity profiles I₂, I₃, I₄ of the second-, third- andfourth-order modes in FIG. 8 are considered in comparison with FIG. 3,then it can be seen that these do not change significantly as a resultof the introduction of the p-doped layer 12. The active zone 1, however,is shifted in the direction of the maximum of the fundamental mode onaccount of the p-doped layer 12, such that the confinement factor CF₁ ofthe fundamental mode is significantly increased.

The confinement factors of the fundamental laser mode and of thesecond-, third- and fourth-order laser modes of the third exemplaryembodiment as a function of the thickness d of the second n-doped layer5 are illustrated in FIG. 9. In this case, the thickness of the p-dopedlayer 12 is kept constant and the thickness of the first n-doped layer 4is varied, such that the thickness of the common waveguide 8 remainsconstant.

FIG. 9 shows that the profile of the confinement factors CF₂, CF₃, CF₄of the higher-order modes remains substantially unchanged, while themaximum of the confinement factor CF₁ of the fundamental laser mode isincreased to approximately 0.85% relative to the first exemplaryembodiment.

This patent application claims the priority of the German patentapplication DE 10 2009 041 934.9, the disclosure content of which ishereby incorporated by reference.

The invention is not restricted by the description on the basis of theexemplary embodiments. Rather, the invention encompasses any novelfeature and also any combination of features, and in particular anycombination of features in the patent claims, even if this feature orthis combination of features itself is not explicitly specified in thepatent claims or exemplary embodiments.

The invention claimed is:
 1. An edge emitting semiconductor lasercomprising: an active layer having a radiation-generating zone; and acommon waveguide configured to guide the radiation generated in theactive layer within the semiconductor laser; wherein the commonwaveguide comprises a first n-doped layer and a second n-doped layer;wherein the second n-doped layer is arranged between the first n-dopedlayer and the active layer; wherein a refractive index n2 of the secondn-doped layer is greater than a refractive index n1 of the first n-dopedlayer by a refractive index difference value dn; wherein the commonwaveguide is free of a p-doped layer.
 2. The semiconductor laseraccording to claim 1, wherein the common waveguide has a primarywaveguide and a secondary waveguide, wherein the first n-doped layer ispart of the secondary waveguide.
 3. The semiconductor laser according toclaim 1, wherein the common waveguide further includes a first undopedlayer arranged between the active layer and the second n-doped layer,wherein a refractive index n3 of the first undoped layer is greater thanthe refractive index n2 of the second n-doped layer.
 4. Thesemiconductor laser according to claim 3, wherein the common waveguidehas a second undoped layer arranged on a side of the active layer whichlies opposite the first undoped layer.
 5. The semiconductor laseraccording to claim 4, wherein the first undoped layer and the secondundoped layer have different thicknesses.
 6. The semiconductor laseraccording to claim 1, further comprising an n-doped cladding layer and ap-doped cladding layer.
 7. The semiconductor laser according to claim 1,wherein a fundamental mode of laser radiation emitted from thesemiconductor laser is substantially limited to the second n-dopedlayer.
 8. The semiconductor laser according to claim 1, wherein therefractive index difference dn between the first n-doped layer and thesecond n-doped layer is between 0.03 and 0.1, inclusive.
 9. Thesemiconductor laser according to claim 1, wherein the thickness of thesecond n-doped layer has a value of between 8% and 20%, relative to thethickness of the common waveguide.
 10. The semiconductor laser accordingto claim 9, wherein the thickness of the second n-doped layer is between200 nm and 800 nm, inclusive.
 11. The semiconductor laser according toclaim 1, wherein an n-type dopant in the first n-doped layer rises in astepped manner or continuously towards an outer side of the commonwaveguide.
 12. The semiconductor laser according to claim 1, wherein thesemiconductor laser is based on an arsenide compound semiconductormaterial.
 13. The semiconductor laser according to claim 1, wherein thesemiconductor laser emits laser radiation from the infrared spectralrange.
 14. The semiconductor laser according to claim 1, wherein theactive layer is arranged within the common waveguide.